1. Field of the Invention
The present invention relates to a projection display apparatus, and more particularly to an optical unit for accurately determining an area in which to project illuminating light onto an image forming device, and a projection display apparatus which incorporates such an optical unit.
2. Description of the Related Art
Projection display apparatus employing image forming devices including liquid crystal light valves, digital micromirror devices (hereinafter referred to as “DMD”), etc. have heretofore been widely used for the purposes of presentations and other image display occasions. In the projection display apparatus, the image forming device produces a light beam modulated according to an input image signal, and the modulated light beam is projected in a magnified scale onto a screen. If a light beam applied to the image forming device fails to accurately illuminate an image forming area on the image forming device, then the projection display apparatus suffers problems in that the projected image has its brightness lowered or is shaded. To solve the problems, it is the general practice to apply illuminating light which is surrounded by a certain illuminating margin to the image forming area. If the illuminating margin is large, however, much light is not applied to the image forming area, resulting in a reduction in light utilization efficiency.
JP-A No. 10-115799 discloses a projection display apparatus employing a liquid crystal light valve. The disclosed projection display apparatus has a field lens whose installed position is adjustable for accurately positioning an illuminating light area in the image forming area of the liquid crystal light valve.
Projection display apparatuses employing a DMD incorporate various technologies including a process of displacing the position of the exit end of a light tunnel to position an illuminating light area accurately in the image forming area of the DMD, a process of displacing a planar mirror inserted in an illuminating light path to position an illuminating light area accurately in the image forming area of the DMD, and a process of displacing a lens in an illuminating light path to position an illuminating light area accurately in the image forming area of the DMD.
However, conventional technologies incorporated in the projection display apparatus with the DMD suffer the following problems:
According to the process of displacing the position of the exit end of the light tunnel to position the illuminating light area accurately in the image forming area of the DMD, when the position of the exit end of the light tunnel is displaced, the position of the entrance end of the light tunnel is also displaced simultaneously, tending to bring the light applied to the light tunnel out of alignment with the optical axis of the light tunnel. When the applied light is brought out of alignment with the optical axis of the light tunnel, the projected image has luminance irregularities. Therefore, it is necessary to move the output end of the light tunnel about the point of intersection between the optical axis of the light tunnel and the applied light. Actually, however, it is not easy to provide a structure for moving the output end of the light tunnel about the point of intersection. Even if it is possible to provide such a structure, the projection display apparatus incorporating the structure still has low productivity.
According to the process of displacing the planar mirror inserted in the illuminating light path to position the illuminating light area accurately in the image forming area of the DMD, it is necessary to move the planar mirror from the starting point at the center of the light beam applied to the planar mirror. If another point is used as the starting point, then the starting point varies from direction to direction in which to move the planar mirror, resulting in a change in the length of the optical path up to the DMD. If the length of the optical path up to the DMD changes, then the illuminating light area tends to be blurred, darkening the peripheral edge of the projected image.
According to the process of displacing the lens in the illuminating light path to position the illuminating light area accurately in the image forming area of the DMD, the lens is moved in two directions, i.e., a vertical direction and a horizontal direction, i.e., a direction normal to the vertical direction. Both the image forming area of the DMD and the exit end of the light tunnel are quadrangular in shape and similar in shape to each other. However, the sides of the shapes of the image forming area of the DMD and the exit end of the light tunnel are oriented differently because the illuminating light passes through a three-dimensional light path. If the DMD has an aspect ratio of 4:3, then the shorter sides thereof are oriented vertically and the longer sides thereof are oriented horizontally, whereas both the shorter and longer sides of the shape of the exit end of the light tunnel are inclined to the vertical and horizontal directions. When the lens is moved in the two directions, i.e., the vertical direction and the horizontal direction, i.e., the direction normal to the vertical direction, the illuminating light area on the DMD is moved obliquely, and hence the directions in which the illuminating light area is moved deviate considerably from the directions of the shorter and longer sides of the DMD. Even if efforts are made to position the illuminating light area accurately in the image forming area of the DMD, it is not easy to bring the illuminating light area into alignment with the image forming area of the DMD. Consequently, the efficiency with which to achieve such area adjustment is low, and it is difficult to reduce the period of time required to perform the positional correction for the illuminating light area.